We are familiar with MC's increasing the overall percentage - across the board, which sometimes happens weeks, even months later.
But arbitrarily switching a particular week's percentages based on the upside-down attendance for a particular movie has never happened to us before.

When the shooting occurred in Colorado, the attendance for all subsequent weeks were better than the first (for all our movies), and we didn't incur any percentage switching.

Their normal is increasingly becoming "Charge as we see fit, whenever and however we choose".

Which is why I tried as much as possible to get the terms in writing by email where possible. Usually no problems except: 1) Lionsgate/Summit did not tell anyone I knew what the terms where on Hunger Games, 2) Disney waiting a long time to settle but expecting payment weeks before, and 3) My Fox rep would NEVER tell me terms by email only over the phone.

I never experienced any changes in terms like you are talking about in 6 years besides the higher % being attributed to the highest grossing week, the 2nd highest % being charged to the 2nd highest week, etc. That was rare. Think it only happened twice to me.

This is known as best weeks. The only one I've ever had enforce it was Sony. Basically - the 70 / 60 / 50 / 40 is not based on chronological weeks, but rather, in the order of highest gross (which usually coincides with chronological). This is not changing terms after the fact. Read your MLA.